The diagram shows the structure of insulin. From the diagram, what can be concluded about the structure of insulin?
[VISUALIZATION: Diagram showing two polypeptide chains (A and B) linked by disulfide bonds, with an additional intrachain disulfide bond in chain A. Amino acid sequence shown.]

• A

  It is composed of two polypeptide chains stabilized by disulfide bonds.

• B

  It is a simple protein composed of one continuous polypeptide chain.

• C

  It is a fibrous protein.

• D

  Its molecules do not display quaternary structure.